Satellite Encounters

GENERAL INFORMATION

With over 9000 cataloged (known) objects in Earth orbit, sooner or later
there will be close encounters (or even collisions) between two satellites.

The first publicly documented collision between two cataloged space objects
occurred in July of 1996. A French military micro-satellite called Cerise ( International
Designation 1995-033B / Satellite
Catalog No. 23606) suddenly lost stability when it appeared that its
stabilization boom was impacted. After analysis, it was concluded that the
possible culprit was a piece of space debris from an Ariane booster
(1986-019RF/18208). Controllers were able to reprogram the payload and regain
attitude control.

Several news releases on Aug 12, 1997 noted that the
CRISTA-SPAS satellite, flying free of STS-85, passed very close to an old
rocket motor launched by the shuttle in 1984 (the payload, a Com sat, failed to
reach the desired geo-synchronous orbit). The satellite in question was not
identified in the news releases. A search of satellites from that shuttle
mission indicated the objects in question were from the shuttle Challenger,
launched in February of 1984.

Doyle Groves (a SeeSat-L
subscriber) using a PC program that can calculate satellite encounters (called
COLA) posted a message
on SeeSat-L indicating that the unidentified satellite in question was most
likely the PAM-D rocket body (14694/1984-011F). The close encounter occurred at
00:40:30.3 UTC on Aug 12 , 1997 at an approximate range of 3.1 km. The
encounter occurred just west of Baha California (Mexico).

Jeremy Ollerenshaw ( a SeeSat-L subscriber), again using
COLA, posted a message
on SeeSat-L indicating that Mir was to have a close pass on Sept 15, 1997 at
20:33 UTC with a US sat MSTI-2
(23101/1994-028A). ABC News later reported on the close encounter (~500 meters)
which took place over northern Japan. A NASA spokesperson said that the
encounter and precautions taken by the Mir crew were not unusual and occur
frequently on a monthly basis. The news report stated this was the closest
encounter in the 11 year Mir history.

Rob Matson (a subscriber to SeeSat-L) reported on
the expected close-encounter of Mir with another satellite rocket body on July
28, 1999 at 18:24:54 UT.

The USSPACECOM
tracks all cataloged satellites for possible collision with the shuttle,ISS,
and US government
maneuvering sats. If it appears an object may come too close to the shuttle,
then the shuttle maneuvers to avoid the object. This has happened with several
missions (STS-44, 48, 53, 72, 82) in which at least 5 maneuvers were required.

The first known attempts to maneuver the ISS are documented in the SeeSat-L
archives here and here. An insight into what is involved to maneuver a satellite (in this case,
the ISS) to avoid a possible collision can be seen here.

The orbit of the ISS was successfully raised for the first time October 26,
1999 to avoid a possible close encounter with another satellite rocket body.

On March 28, 2002 at 20:44 UTC the ISS and a Delta 2 rocket body passed within 14 km of each
other over New Guinea. It was decided beforehand that it would not be necessary
to maneuver the ISS. These orbital elements
can be used with any satellite tracking program to review this encounter.

The Russians have their own tracking systems that include both radars and
optical telescopes. Supposedly, Mir had never maneuvered to increase the
distance between it and a possible encounter object. When a close encounter was
predicted, all the Mir crew retired to the Soyuz vessel until the encounter time
period passed. Of course, Mir had to be periodically maneuvered (usually by
an attached Progress vessel) to maintain its desired altitude. Atmospheric drag
slowly lowered the altitude of Mir.

The problem arises with those objects that can not be tracked because of
their small size. Debris larger than 1 cm in diameter present a catastrophic
hazard to orbiting payloads. In addition, there is no known shielding material
available for debris of this type. It's believed, only the high powered, narrow
angle field of view tracking radars can track objects down to 1 cm in diameter,
up to altitudes of a 1000 km.

An example of how much kinetic energy can be obtained by a small amount of
mass at orbital velocity follows. Aluminum has a density of about 2.66
grams/cubic-cm. A 1-cm diameter sphere of aluminum has a volume of 0.5236
cubic-cm, thus a mass of about 1.4 grams. Assuming a collision velocity of 10
km/sec (NASA says the average is 16 km/sec), the kinetic energy of the
collision would be:

This is the same energy as a 16-lb (7.3 kg) bowling ball impacting at 312
mph (520 km/hr).

More US optical tracking systems with higher detector resolution are being
put into service to supplement the wide angle field of view radars that may
have a resolution to detect objects tens of centimeters in diameter. The USAF GEODSS (Ground based Electro-Optical
Deep Space Surveillance system) has large optical telescopes at Diego Garcia in the Indian Ocean, Maui (Hawaii) and
Socorro (New Mexico) .

The shuttle returns from every mission with minor damage caused by untracked
man-made objects (debris) or natural space-born objects (micrometeoroids). Most
of the damage has occurred on the thermal tiles. Damage has occurred on eight
external panes of the multi-layer viewing windows for the shuttle vehicles
warranting replacement of those external panes.

STS-70 (July 1995) took a hit on window #6 on the forward right side,
nearest the pilots station. The ding was about 1/16 inch in size and occurred
as the shuttle was in a wing velocity vector mode. It probably took place while
the crew were asleep as they noticed it after they woke up. A photo
(177k) originating from NASA-JSC clearly shows the impact area on the removed
window.

Whenever the shuttle isn't required to be in a certain attitude, it will
orbit with the tail section facing toward the direction of flight so as to
minimize damage to critical components.

TRACKING CLOSE ENCOUNTERS

Rob Matson, a subscriber of SeeSat-L , has developed a program for PCs
called COLA (COLlision Avoidance) that can calculate how close a target satellite may
encounter other selected satellites. Y2K compliant copies of his program COLA and ALLCOLA
can be found at Mike McCants'web site.

Rob periodically posts updates to SeeSat-L
regarding either past or future close encounters. An example of some of these
posts can be found here and here and
regarding the ISS, here.